Lithium ion rechargeable batteries are a prime candidate for a variety of devices, including electric vehicle (EV) and hybrid electric vehicle (HEV) applications, due to their high energy capacity and light weight. All cells are built from a positive electrode (cathode) and a negative electrode (anode), electrically isolated by a thin separator and combined with a liquid transporting medium, the electrolyte. Typically, the anode of a conventional Li-ion cell is a composite electrode including at least one active material, i.e., carbonaceous materials and/or silicon based materials, a conductive additive, and a polymeric binder, the cathode is typically a composite electrode too, with a metal oxide as the active material, a conductive additive, and a polymeric binder, and the electrolyte. Both the anode and the cathode contain active materials into which lithium ions insert and extract. The lithium ions move through an electrolyte from the negative electrode (anode) to the positive electrode (cathode) during discharge, and in reverse, from the positive electrode (cathode) to the negative electrode (anode), during recharge.
Electrode design has been a key aspect in achieving the energy and power density, and life performance required for electric vehicle (EV) batteries. State-of-art lithium-ion electrodes have used a polymer binder to ensure the integrity of the composite electrode for a dimensionally stable laminate. The polymer binder plays a critical function in maintaining mechanical electrode stability and electrical conduction during the lithium insertion and removal process. Typical binders which can be used are starch, carboxymethyl cellulose (CMC), diacetyl cellulose, hydroxypropyl cellulose, ethylene glycol, polyacrylates, poly(acrylic acid), polytetrafluoroethylene, polyimide, polyethylene-oxide, poly(vinylidene fluoride) and rubbers, e.g., ethylene-propylene-diene monomer (EPDM) rubber or styrene butadiene rubber (SBR), copolymers thereof, or mixtures thereof. Typically, the anode and the cathode require different binders. For example, styrene-butadiene rubber (SBR) is a binder which is mainly used to prepare the anode electrode. Polyvinylidene difluoride (PVDF) is mainly used to prepare the cathode electrode. In addition, classic electrode materials such as lithium cobalt oxide (LiCoO2) and graphite powder are known to provide dimensional stability during the electrochemical processes. The polymer binder materials such as polyvinylidene difluoride (PVDF) are often used to adhere to electron conducting particles in maintaining the physical contacts for electrical connection within the laminate.
This state-of-the-art approach works fairly well until the introduction of higher-capacity electrode materials such as silicon (Si) in the composite electrode. Silicon (Si) materials have been extensively explored as one of the most promising anode candidates for lithium-ion batteries because of its ability to provide over ten times greater theoretical specific capacities than conventional graphite based anodes. Additionally, because silicon is abundant, it is less costly to use when compared to other alternatives for high energy lithium-ion battery application. However, Si volume change during cycling has created excessive stress and movement in the composite electrode and increased surface reactions. Specifically, electrochemical alloying of Li with Si gave Li4.4Si as the final lithiation state and a capacity of close to 4,200 mAh/g. However, almost 320% volume expansion occurs as the material transitions from Si to the Li4.4Si phase during charging. Because of this high volume change, the electronic integrity of the composite electrode is disrupted, and a strong and continuous surface side reaction is induced, both leading to a fast capacity fading of the battery, and overall decreased battery life.
In order to use Si material, a new method to assemble Si-active material articles must be put in place, along with Si surface stabilization. With the in-depth knowledge of the Si surface properties and increased commercial supply of Si for battery applications, there is an opportunity/demand to investigate better Si assembly and stabilization for electrode application.
Accordingly, it is an object of the present invention to overcome, or at least alleviate, one or more difficulties and deficiencies related to the prior art. These and other objects and features of the present invention will be clear from the following disclosure.